The present invention is directed to a tie member including at least one tension element, such as a steel wire, a steel strand or the like, enclosed within a tubular sheathing and secured at its opposite ends in anchoring units for transmitting tension force to part of a structure. The anchoring units include an anchor member or plate containing at least one conically shaped borehole with the tension element being anchored in the borehole by a multi-part annular wedge.
Such a tension tie member can be used as a prestressing member for prestressed concrete where an individual prestressing member includes a tension element or a bundle of such elements which may or may not be in composite or bonded action with part of a structure. Alternatively it may be in the form of a tie rod tensioned between and anchored to parts of a structure, such as a diagonal cable for a stayed girder bridge.
Prestressing members for prestressed concrete consist of one or more tension elements guided in part of a structure within a sheathing tube so that it can be moved in its long direction and tensioned after the concrete forming the structure has set and then anchored on a part of the structure. The individual tension elements can remain without any bonding to the part of the structure and, accordingly, can be post-tensioned or can be placed in bonded or composite action with the structure by grouting a hardenable material about the tension element.
Tie rods such as used in civil engineering for anchoring parts of a structure, such as diagonal cables for stayed girder bridges and the like, often are made up of a bundle of individual tension elements, such as steel wires or strands, arranged together in an unsupported region of the tie member within a tubular sheathing. The ends of the tie member are guided through different parts of the structure and anchored on the opposite side of the part from which the tie member enters the structure. Anchoring units for the tie rods include an anchor member, such as an anchor disc or plate, with conically shaped boreholes through which the individual tension elements are inserted and in which they are anchored individually by multi-part annular wedges. One problem experienced with such tie members is that the anchoring units, based on the principle of wedge anchoring, have only a relatively limited fatigue strength and, as a result, are sensitive to fatigue failure. When a tensioned element is anchored, the annular wedges, made up of several wedge sections, are drawn into a conically shaped borehole in the anchor member due to the tensile force acting in the axial direction of the tension element. Clamping forces acting perpendicularly to the axis of the element are produced by the wedge sections and these clamping forces prevent movement of the tension element. The concept underlying such anchorages is that the friction coefficient between the tension element and the wedge is greater than the friction coefficient between the wedge and the conical borehole. As a result, the inside surfaces of the wedge segments are provided with a shaped surface in the form of fine teeth so that the wedge can bite into the surface of the tension element. The teeth are formed by cutting a fine thread in the inside surface of the conically-shaped wedge member before it is divided into the individual wedge sections.
Nevertheless, when dynamic loads are experienced in the structure, such as live loads in a bridge, certain movements, though very limited, take place in the region of the wedge anchorages. Due to such movement friction corrosion can take place when oxygen contacts the tension element with friction corrosion developing and leading to premature failure of the tension elements due to fatigue.
In tie members tensioned between parts of a structure, such as diagonal cables in stayed girder bridges, the tubular sheathing in the unsupported portion of the tie member may be formed of a plastics material tube of polyethylene, or a steel tube. Usually, a steel anchor tube is provided in the anchoring region to absorb the deflection forces which develop when the tension elements are spread as they move toward the anchorage. The open space within the tubular sheathing between the tension elements is filled with an anticorrosive substance, such as grease, or with a hardenable material, such as a cement mortar or a synthetic resin, to protect the tension elements from corrosion. A tie rod of this type can be post-tensioned or replaced after the filling or grouting step.
While thick-walled steel tubes as sheathing in the unsupported region of the tie member can afford the tension elements with good corrosion protection, such tubes cannot be produced in the full length of the tension member and, therefore, must be welded together at joints. Weld seams or joints, however, form weak points where cracks or fractures may occur as a result of fatigue under alternating loads. Plastics material sheathing tubes, such as polyethelene tubes, avoid these problems, however, they are not vapor-tight. Accordingly, such tubes do not provide sufficient corrosion protection for the tension elements within the sheathing if the cement mortar or grout filling the space around the elements happen to develop cracks. The same situation is true for longitudinally seamed, helically wound or longitudinally and transversely welded sheet metal tubes, because such tubes are not absolutely tight in the seams or at the joints or because of possible damage at other locations.
Finally, when the tie member is used as a diagonal cable for stayed girder bridges, the tension elements are left ungrouted for long periods of time, since the final tension force on the cables can only be applied after the entire bridge has been completed. If the space around the tension elements is grouted with a hardenable material, any post-tensioning or relaxing of the tension force which may be required will be made more difficult. Accordingly, a temporary corrosion protection must be provided at the construction site.